In various application fields, the demand for high performance polyelectrolyte membrane is growing. Particularly, in the filed of fuel cell, since the application thereof to portable, home and automotive uses, such as mobile PC, PDA, and cellular phone, is expected, the development of high performance membrane is urgently required. At first, a fluorine-containing electrolyte membrane has been mainly used. The fluorine-containing electrolyte membrane is excellent in ion conductivity and certain durability, but involves drawbacks, e.g., generation of fluorine compounds due to the decomposition during its use, high fuel permeation, and high costs. Therefore, the substitute for fluorine-containing electrolyte membrane has been demanded.
Recently, hydrocarbon materials have been proposed as a substitute for fluorine-containing electrolyte. Specifically, a material produced by introducing ion-conducting groups such as sulfonic acid group to a base polymer having a nature resembling engineering plastic, for example, polyether sulphone (PES) and polyether ether ketone (PEEK) has been proposed.
For example, Patent Document 1 proposes a sulphonated PES. Since such material is free from fluorine, the fluorine compound cannot be generated even when the material is degraded. Such material would be costly advantageous if the technical problems in its production, for example, the introduction of ion-conducting groups and the film formation, are solved.
However, since the engineering plastics are basically random polymers, the introduced ion-conducting groups are distributed relatively uniformly. High ion exchange capacity is required for obtaining high ion conductivity. However, the swelling tends to easily occur as the ion exchange capacity is increased. If a material having high ion exchange capacity is used as the electrolyte membrane of a fuel cell, the membrane is subjected to swelling-contracting cycles repeatedly during repeated start-stop cycles (repeated humidifying-drying cycles) and causes cracks because the base polymer is hard and brittle, thereby likely to cause the leakage of fuel. Since the base polymer is particularly hard and brittle in absolutely dried condition, a membrane having ion conductivity sufficient for practical use has not yet been obtained.
To prevent the swelling and enhance the water resistance, a sulphonated product of modified PES comprising blocks having ion-conducting groups introduced and blocks not introduced has been proposed. However, the proposed material is produced basically by polycondensation and therefore the blocks are randomly mixed, failing to obtain a sufficient phase separation (Patent Document 2).
The electrolyte membrane described in Patent Document 3 has good bonding ability with electrodes because it is made of a block copolymer having flexible blocks. In addition, since the block copolymer is not a polycondensed material, the block structure is maintained after the polymerization. Therefore, the ion channel and the rubbery portion are kept completely apart from each other by the phase separation structure peculiar to block copolymers, to provide a membrane capable of preventing cracks due to repeated humidifying-drying cycles.